magnetic control electric and reversal behavior of zno nife zno multilayer films

Magnetic-control-electric and reversal behavior of ZnO/NiFe/ZnO multilayer films Po-Wei Chi, Da-Hua Wei, Chin-Chung Yu, and Yeong-Der Yao Citation: AIP Advances 7, 056309 (2017); doi: 10.1063/1.4975049 View online: http://dx.doi.org/10.1063/1.4975049 View Table of Contents: http://aip.scitation.org/toc/adv/7/5 Published by the American Institute of Physics AIP ADVANCES 7, 056309 (2017) Magnetic-control-electric and reversal behavior of ZnO/NiFe/ZnO multilayer films Po-Wei Chi,1 Da-Hua Wei,1,a Chin-Chung Yu,2 and Yeong-Der Yao3 Institute of Manufacturing Technology and Department of Mechanical Engineering, National Taipei University of Technology (TAIPEI TECH), Taipei 10608, Taiwan Department of Applied Physics, National University of Kaohsiung, Kaohsiung 81148, Taiwan Institute of Physics, Academia Sinica, Taipei 11529, Taiwan (Presented November 2016; received 29 August 2016; accepted November 2016; published online 25 January 2017) The magnetic-control-electric and corresponding dielectric behavior of the ZnO/NiFe/ZnO multilayer films have been demonstrated by applying an ultrathin bimetallic NiFe inserting layer into ZnO films, and fabricated by radio-frequency magnetron sputtering at room temperature without introducing any oxygen gas during deposition process At first, a high quality crystalline ZnO(002) textured film was deposited and exhibited a dielectric constant value of around 10 confirmed at room temperature with the Agilent 42941B probe and 4294A impedance meters ranged from 40 Hz to 20 MHz Once ZnO inserted with a nm-thick NiFe inserting layer, the value of dielectric constant was dramatically increased from 10 to 12.5 This phenomenon can be attributed to redistribute the strongly interface charges between ZnO and NiFe layers and accompany with the relaxation of internal stress of ZnO On the other hand, the external magnetic field induced dielectric variation can also be clearly observed, and the ZnO film with NiFe inserting layer demonstrates a 0.05%-0.10% dielectric tunability The magnetic-control-electric and corresponding dielectric behavior of ZnO/NiFe/ZnO multilayers with a single inserting NiFe layer compared with that of pure ZnO film also conclude the magnetoelectric effect in present multilayered structures Moreover, the grain size of the ZnO films was gradually increased from 32.5 nm to 40.5 nm while inserting with an ultrathin NiFe bimetallic layer This grain structure transition can be attributed to the lattice misfit between ZnO and NiFe This research work demonstrates that a single NiFe insering layer can effectively control the dielectric and magnetic characters in the ZnO/NiFe/ZnO multilayered structures and provide valuable multifunctional behaviors for potential novel applications design such as ferroic sensor © 2017 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4975049] INTRODUCTION Recently, polar Zinc oxide (ZnO) has been regarded as an important candidate due to its direct wide band gap (3.37 eV), high exciton binding energy (60 meV), optical transparent for visible light Moreover, transition metal-doped polar ZnO is predicted to act as a semiconductor with ferromagnetic behavior at room-temperature (RT) overcoming the limitation of Curie temperature (Tc) Therefore, polar ZnO is an expected material for many novel applications such as piezoelectric transducers, transparent thin film transistors, chemical gas sensors or biosensors,1–3 solar cells, ultraviolet (UV) detectors4–6 and spin light-emitting diode (LED).7 Another attraction of ZnO is in its physical property that can be modulated by ferromagnetic materials.8–10 As it has been known in the development of multiferroic materials, they have been attracted much attention continuously due to a Corresponding author E-mail: dhwei@ntut.edu.tw 2158-3226/2017/7(5)/056309/7 7, 056309-1 © Author(s) 2017 056309-2 Chi et al AIP Advances 7, 056309 (2017) the coexistence of ferromagnetic and ferroelectric properties Therefore, the modern control type such as electrical-driven nanodevices11 have been widely explored and used in storage medium12 and field of spintronics.13 In other words, it is also expected to control the electric properties by an applied magnetic field For above various suitable applications of ZnO, the combination of ZnO with other functional materials has attracted considerable interest, especially combine with some magnetic metals This idea leads not only to scientific interest but also technological concern with its magnetoelectric property On the other hand, the Ni80 Fe20 (permalloy) has excellent soft magnetic properties and widely used in magnetic-based electronic device such as magnetoresistive random-access memory (MRAM).14 It is interesting that the electrical and magnetic property could be changed with the various composite ratios of nickel and iron For instance, the content of nickel is 35% and named as permalloy, which exhibits high electrical resistivity (ρ) but low susceptibility On the other hand, the contents of nickel are 50% and 80%, which exhibit high saturation magnetization (Ms) and low coercivity without any magnetostrictive effect.15 In this present research work, we observed the enhancement of dielectric properties of ZnO thin film with an ultrathin NiFe bimetallic inserting layer under the frequency ranged from 40 Hz to 20 MHz The crystalline structure for pure ZnO and ZnO/NiFe/ZnO multilayer films were also characterized The extrinsic properties for ZnO/NiFe/ZnO multilayer films such as dielectric constant and tunability as a function of frequency were measured at room temperature by varying external magnetic fields This research work not only extends the scope of potential magnetoelectric applications for ZnO/NiFe/ZnO multilayer structures but also provides the potential development of magnetic-driven nanodevice EXPERIMENTAL PROCEDURES The inset graph in Fig shows the schematic illustration of multilayered film structures in this work The ZnO (50 nm)/NiFe (5 nm)/ZnO (100 nm) multilayer films deposited onto D263T glass substrates were employed by a radio-frequency (RF) magnetron sputtering system, and all the substrates were placed parallel to the bimetallic NiFe and ceramic ZnO targets The NiFe target is composed of Ni 80% and Fe 20%; the ZnO target is composed of 99.99% purity pressed ZnO powder, and all targets size are with 0.075 m diameter and 0.006 m thickness, respectively All the substrates were rinsed in deionized water, ultrasonically cleaned in ethanol and acetone to remove organic contamination then dried in hot air before loading into the sputtering vacuum chamber The sputtering chamber was pumped down to a base pressure of × 10☞7 torr Argon was filled into sputtering chamber sequentially with the low working pressure of mtorr The NiFe thin films were FIG The permittivity spectrum for the ZnO/NiFe/ZnO multilayer films without and with an ultrathin NiFe inserting layer at frequency ranged from 40 Hz to 20 MHz The inset shows the schematic diagram illustrations of the ZnO (50 nm)/NiFe (5 nm)/ZnO (100 nm) multilayer structures 056309-3 Chi et al AIP Advances 7, 056309 (2017) deposited with direct current (DC) power fixed at 15 W and the ZnO thin films were deposited with RF power fixed at 100 W, respectively The deposition rates of NiFe and ZnO were 1.8 and 5.6 nm/min, respectively The crystalline structure of ZnO/NiFe/ZnO multilayer films was characterized by ex-situ X-ray diffraction (XRD) with Cu Kα radiation (λ = 1.54 Å) in the range of 2θ = 30-50o The dielectric properties for all samples were measured at room temperature with the Agilent 4294B probe and 4294A impedance meters ranged from 40 Hz to 20 MHz without and with a maximum applied field of 100 Oe parallel to the film plane The magnetic hysteresis loops were investigated at room temperature using the magneto-optical Kerr effect (MOKE) RESULTS AND DISCUSSION The variation of dielectric constant as a function of frequency ranged from 40 Hz to 20 MHz for pure ZnO and ZnO/NiFe/ZnO multilayer films with a nm-thick NiFe inserting layer are shown in Fig It can be clearly observed that dielectric constant of all samples slowly decreased with increasing the frequency from 40 Hz to 20 MHz In addition, the dielectric constant for the pure ZnO film with thickness of 150 nm at frequency of MHz is around 10 as shown in Fig While inserting with a nm-thick NiFe layer, the value of dielectric constant was dramatically increased from 10 to 12.5 It is suggested that dielectric constant of the ZnO/NiFe/ZnO multilayer films gets prompt improvement Practically, the interfacial polarization resulting from the charge redistribution between the ZnO and NiFe make a contribution in dielectric properties at high frequency According to the trend of Fig 1, it can be realized that there are more than one type of polarization mechanisms in pure ZnO and ZnO/NiFe/ZnO multilayer films For pure ZnO films, a larger dielectric constant value of around 31 can be observed in the low frequency regions, and this phenomenon is due to the space charges move under this field and trapped at the interfaces between defects then forming many dipole moments Therefore, space charge polarization is dominant in the low frequency regions, and this type of polarization mechanism may be explained on the basis of the Maxwell-Wagner model of dielectric behavior.16,17 On the other hand, the orientation polarization is dominant in the high frequency regions (over 104 Hz), and the rotational displacement of dipoles in the ZnO films results in this orientation polarization Therefore, pure ZnO film has a large dielectric constant value around 31 in the low frequency regions, which decreases with increasing frequency and attains a constant value in higher frequency regions Moreover, an interesting result of dielectric property can be observed in ZnO/NiFe/ZnO multilayer films While inserting a nm-thick NiFe bimetallic layer, the dielectric constant (read at 40 Hz) obviously increased from 31 to 36 However, the crystalline structure of material maybe another important factor that affects the variation of dielectric property Therefore, X-ray diffraction analysis is used to confirm the crystalline structure of pure ZnO and ZnO/NiFe/ZnO multilayer films In order to realize the dielectric enhancement of the ZnO/NiFe/ZnO multilayer films, the crystalline structure was characterized by ex-situ X-ray diffraction (XRD) as shown in Fig From the XRD patterns, it can be clearly observed that both samples exhibit a strong peak located at around 2θ = 34◦ originated from 150 nm thick ZnO film After inserting a nm-thick NiFe layer, XRD patterns show the peaks of ZnO and NiFe located at around 2θ = 34◦ and 43◦ , which are corresponded to the ZnO (002) plane (JCPDS Card: 361451) and NiFe (111) plane (JCPDS Card: 471417), respectively Above results indicate that the ZnO/NiFe/ZnO multilayer films are highly textured growth, and the NiFe and ZnO are in face-centered cubic (fcc) and hexagonal close-packed (hcp) phase structures, respectively The strong signal intensity of the (002) diffraction peak from the (002) plane is due to the lowest surface energy of the (002) basal plane in ZnO phase, leading to a preferred orientation along the [001] crystalline direction.18 Moreover, it can be observed that the (002) diffraction peak shifted from low angle degree (34.04◦ ) to high angle degree (34.16◦ ) as shown in the XRD patterns, indicating the internal stress state of the multilayer films changed while inserting a NiFe layer into ZnO According to previous reports,19,20 the in-plane biaxial stress in ZnO can be obtained from the following equation: c0 − c GPa, (1) σ = 450 c0 056309-4 Chi et al AIP Advances 7, 056309 (2017) FIG XRD patterns for the ZnO/NiFe/ZnO multilayer films deposited onto glass substrates at room temperature without and with an ultrathin NiFe inserting layer, respectively where c and c0 are the c-axis lattice constant evaluated from Bragg’s law (2dsinθ = nλ) and the c-axis lattice constant for strain free bulk ZnO, respectively The parameter σ is the residual in-plane biaxial stress calculated from the out-of-plane stress Figure shows the internal stress value of the pure ZnO and ZnO/NiFe/ZnO structures Obviously, the stress values of the samples are ☞4.41 and ☞3.54 GPa, respectively Above results indicated that both samples show a compressive stress state, and the internal stress is found to be slightly released after inserting with a NiFe layer This internal stress relaxation can be attributed to the lattice misfit between ZnO and NiFe The lattice constant of fcc NiFe (3.59 Å) is larger than the hcp ZnO (3.25 Å) It is implied that tensile stress along a-axis of ZnO is preferred to expand the a-axis and contract the c-axis leading to enhance dielectric property The similar result has been observed in the BaTiO3 /SrTiO3 superlattice structure, which was fabricated on SrTiO3 (001) single-crystal substrate and abnormally enhanced its dielectric property.21 This explains why the ZnO/NiFe/ZnO multilayer films exhibit a higher dielectric constant compared with that of single ZnO film due to the stress modification by addition of a single NiFe inserting layer On the other hand, this kind of internal stress relaxation not only enhanced dielectric property but also induced the grain size growth.22 Figures 4(a) and 4(b) show the top view FE-SEM photos of pure ZnO and ZnO/NiFe/ZnO multilayer films A clear variation in surface morphology can be observed and the average grain size increased while inserting a nm-thick NiFe layer Figures 4(c) and 4(d) present the grain size histograms for evaluating average size and its distribution of ZnO without and with a single NiFe FIG Lattice constants and the internal stress for the ZnO/NiFe/ZnO multilayer films without and with ultrathin an ultrathin NiFe inserting layer 056309-5 Chi et al AIP Advances 7, 056309 (2017) FIG Top view FE-SEM images for the ZnO/NiFe/ZnO multilayer films (a) without and (b) with an ultrathin NiFe inserting layer, respectively (c) and (d) show the corresponding grain size histograms for evaluating average size and its distribution bimetallic inserting layer The grains size in fraction of pure ZnO thin film with 150 nm thick ranged from 22 to 43 nm, with an average size of 32.5 ± 10.5 nm as shown in the Fig 4(c); the grains size in fraction of inserting NiFe thin film with nm thick ranged from 28 to 53 nm, with an average size of 40.5 ± 12.5 nm as shown in the Fig 4(d) Above results could be attributed to relaxation of internal stress of ZnO after inserting with a NiFe single layer that lead to the transformation of grain morphology and its size Therefore, the microstructure of the ZnO/NiFe/ZnO multilayer films could be controlled by suitably inserting with an ultrathin NiFe layer It is well-known that N80 iFe20 magnetic alloy is a ferromagnetic material and possesses a manifest characteristic of soft magnetic hysteresis The ZnO/NiFe/ZnO multilayer films with a nm-thick NiFe inserting layer is characterized by MOKE measurement at room temperature under an in-plane applied longitudinal field As shown in Fig 5(a), the ZnO/NiFe/ZnO sample is with the coercivity value of 30 Oe and magnetic squareness ratio of 0.88, respectively Using the impedance probe and external magnetic field ranged from to 100 Oe, the dielectric spectrum relative to an external magnetic bias is recorded by the impedance meter Before the measurement of dielectric constant, the single ZnO film is preliminarily measured by the impedance meter sweeping the external magnetic field from to 150 Oe The single ZnO layer demonstrates a nearly constant dielectric property and cannot be affected by the external magnetic field as shown in Fig 5(b) The ZnO/NiFe/ZnO multilayered structure was measured by this standard with the same process Once ZnO/NiFe/ZnO multilayered structure was applied with a magnetic field variation, the ZnO film with a single NiFe inserting layer could have a 0.05%-0.10% dielectric tunability as shown in Fig 5(b) Comparing with the single ZnO film, the ZnO/NiFe/ZnO multilayer films with a NiFe inserting layer have obviously enhanced dielectric property Moreover, the dielectric property of ZnO/NiFe/ZnO structures can be reversed back to initial state while removing the external magnetic field Above information show that dielectric 056309-6 Chi et al AIP Advances 7, 056309 (2017) FIG (a) In-plan normalized magnetic hysteresis loop of ZnO/NiFe/ZnO multilayer films with an ultrathin nm thick NiFe inserting layer (b) The relationships between the dielectric tunability and the external magnetic field for ZnO/NiFe/ZnO multilayer films without and with an ultrathin NiFe inserting layer, respectively tunability can be greatly enhanced while inserting with a nm-thick NiFe layer The dielectric data variation from the ZnO/NiFe/ZnO multilayered film was also associated with the magnetoelectric coupling effect existed in the ZnO/NiFe/ZnO structures According to results reported by Alexe et al.,23 the magnetoelectric coupling can be induced by a non-centrosymmetric Fe2+ /Fe3+ charge ordering in single-phase materials such as Fe3 O4 and LuFe2 O4 24 However, this finding suggests that magnetoelectric behavior can be improved under external magnetic field caused by the alternation of Fe2+ and Fe3+ ions and induced the charge ordering at the interface of ZnO/NiFe heterojunction.25–27 CONCLUSIONS The ZnO/NiFe/ZnO multilayer films with ferromagnetic and dielectric properties have been successfully demonstrated by inserting an ultrathin NiFe bimetallic layer into ZnO film at room temperature High quality crystalline ZnO(002) textured film was fabricated at first and displayed a dielectric constant value of around 10 at frequency of MHz confirmed at room temperature Once inserting an ultrathin NiFe bimetallic layer into ZnO, the value of dielectric constant was increased from 10 to 12.5 This phenomenon can be attributed to redistribute the strongly interface charges between ZnO and NiFe layers and accompany with the relaxation of internal stress of ZnO On the other hand, the ZnO/NiFe/ZnO films displayed a higher dielectric constant than the single ZnO film in the low frequency regions Moreover, the external magnetic field induced dielectric variation can be clearly observed, and the ZnO film with NiFe inserting layer demonstrates a 0.05%-0.10% dielectric tunability After removing the external magnetic field, the dielectric property can be reversed back to initial state We present ZnO/NiFe/ZnO multilayer films that indicated the magnetoelectric effect and can be tunable in this kind of designed multilayered structures This research work demonstrates that direct a single 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(2017) Magnetic- control- electric and reversal behavior of ZnO/ NiFe /ZnO multilayer films Po-Wei Chi,1 Da-Hua Wei,1,a Chin-Chung Yu,2 and Yeong-Der Yao3 Institute of Manufacturing Technology and Department... confirm the crystalline structure of pure ZnO and ZnO/ NiFe /ZnO multilayer films In order to realize the dielectric enhancement of the ZnO/ NiFe /ZnO multilayer films, the crystalline structure was... observed, and the ZnO film with NiFe inserting layer demonstrates a 0.05%-0.10% dielectric tunability The magnetic- control- electric and corresponding dielectric behavior of ZnO/ NiFe /ZnO multilayers